Silicon nitride is mainly applied in high temperature materials. It has excellent high temperature strength, as well as other excellent properties such as heat resistant, corrosion resistant, wear resistant, anti-shock and pressure resistant. Moreover, its high mechanical strength can be compared with metal materials. Meanwhile, it is also a high functioning electrical insulating material that has great potential to be utilized in fields such as manufacturing cutting tools, ceramic bearings, refractory materials, high-frequency components and semiconductors.
Silicon nitride exists in α and β phases, which are both in hexagonal crystal system and have similar phase like unit cell. Generally, a phase silicon nitride is a low temperature type which is unstable and often contains traces of oxygen, the empirical formula of which is Si12N15O0.5. β-silicon nitride is a product of higher temperature and low oxygen partial pressure. As such, when the temperature exceeds 1650□, α-silicon nitride can transform directly to high length-diameter ratio column-like β-silicon nitride grains. The non-uniform distribution of such grains can create crack bridging, which is the main reason behind high-strength and high-toughness of the sintered silicon nitride. Theoretically, the density value of α-silicon nitride and β-silicon nitride is 3.18 and 3.19 g/cm3, respectively and a significant thermal decomposition of silicon nitride occurs at around 1800° C. under nitrogen gas at one atmosphere pressure. When silicon nitride is subjected to atmosphere or higher oxygen partial pressure, a protective layer of silicon dioxide is formed to inhibit oxidation reaction.
Several methods are employed in preparing silicon nitride powders in the industry, including: direct nitridation, chemical vapor deposition, imide thermal decomposition, combustion synthesis, carbothermal reduction and etc., wherein carbothermal reduction is the main method adopted in preparing silicon nitride in the industry. Carbothermal reduction involves solid phase mixing of silicon oxide or silicon dioxide powders with carbon sources and subjected to powder sintering process under nitrogen gas or ammonia gas at a high temperature furnace wherein carbothermal reduction process is carried out. The powder fabricated using such method are fine powders and also has the advantages of having even particle size, high purity and is capable of large scale production and thus besides direct nitridation process is also a primary method of producing silicon nitride powder in the industry.
The reaction for producing silicon nitride powder is listed below:nitrogen gas: 3SiO2(s)+6C(s)+2N2(g)→Si3N4(s)+6CO(g) ammonia gas: 3SiO2(s)+6C(s)+4NH3(g)→Si3N4(s)+6CO(g)+6H2(g) 
ZHONG XIAN-LONG et al, Taiwan patent No. 1347299, entitled “Method for manufacturing [alpha] phase silicon nitride powder with high specific surface area” discloses a method for manufacturing [alpha] phase silicon nitride powder with high specific surface area, which is performed by using NH4NO3 which serves as oxidizing agent and glycine and urea which serve as fuels to carry out carbothermal combustion nitridation reaction to produce a reaction precursor powder (SiO2+C), however, as the carbon ratio between precursor and silicon dioxide is too low, sucrose is additionally added as a carbon source, lastly the obtained precursor is subjected to carbothermal reduction nitridation carried out in a column high temperature furnace under nitrogen gas.
SHIBATA KOJI et al., Taiwan patent No. I573757, entitled “Silicon nitride powder production method, silicon nitride powder, silicon nitride sintered body and circuit substrate using same” discloses a method to produce a silicon nitride powder with low oxygen content inside and an oxygen surface suitable for sintering, whereby thermal decomposition of nitrogen-containing silane compounds including silicon diimide, silicon tetramide, silicon chloroimide is carried out to obtain amorphous Si—N—H compound, and sintering process is carried out by placing the compounds in a continuous sintering furnace at a flow-state, under nitrogen gas at a temperature range from 1400-1700° C.
Crosbie, U.S. Pat. No. 4,582,696, entitled “Method of making a special purity silicon nitride powder” discloses a method of making high purity α-silicon nitride powder which involves a combustion reaction of tetraethyl orthosilicate and ammonia gas to produce an amorphous silicon powder and carbon black, followed by carbonthermal reduction nitridation at 1300-1500° C. under nitrogen gas, to produce a high purity α phase silicon nitride powder.
Schroll, U.S. Pat. No. 8,697,023, entitled “Method for producing high-purity silicon nitride” discloses a method of making high purity silicon nitride powder, which involves placing high purity silicon powder in a rotational column high temperature furnace, through controlling the mixing ratio among nitrogen gas, argon gas and hydrogen gas, a sintering process is carried out at 1100-1450° C. to complete nitridation reaction, so as to obtain high purity silicon nitride powders.
Commonly, direct nitridation process and carbothermal reduction are adopted as two primary methods to synthesize silicon nitride powders. However, direct nitridation process is both time consuming and power inefficient. Therefore, the present invention mainly adopts carbothermal reduction method to produce a silicon nitride powder. Carbothermal reduction mainly involves mixing silicon dioxide powder with carbon black and the carbonthermal reduction nitridation is carried out under nitrogen gas at 1500° C. However, it is difficult to evenly mix the silicon dioxide powder with the carbon black, which can undesirably result in incomplete carbonthermal reduction nitridation of the powder and thereby leading to a problem with too much carbon residues, such that the time required for subsequent carbon removal process is prolonged. As such, the oxygen content of the silicon nitride is increased and the purity of which is decreased, thereby adversely affecting the block molding quality and reliability.